Post date: Mar 22, 2019 2:05:48 PM
I received an email from a peer, that they did not reproduce my results of FeO in PRB 96, 100101(R). The reason is that the Jahn-Teller distortion cannot be automatically captured even with DFT+U and HSE. This may be commonly encountered and ignored by beginners, and actually I know a journal paper showing the HSE results unfortunately with the same pitfall. Below is my reply (slightly modified), and hope it will help.
FeO is tricky to calculate even with PBE+U and HSE. I have found one paper with HSE (not cited in my paper), which opens the band gap, distorts the structure, but the result is still not right since the ground state was not found. One may found something strange in the DOS plot. For HSE with the high energy structure, the Fe d states near the VBM is not a peak but a tail, and the band gap is much smaller than the ground-state structure.
With PBE+U and SCAN+U, the distorted yet not-ground-state structure may even be metallic. The reason is that the rhombohedral distortion is mainly arising from the lower symmetry due to the magnetic configuration (compared to the RS structure), while the large Jahn-Teller distortion is not (fully) captured. The large Jahn-Teller distortion can be confirmed by looking at the Fe-O bond lengths, and for FeO two of the six bonds are much shorter.
For PBE and/or PBE+TS without U, the band gap is not open, and hence using an initial structure containing Jahn-Teller distortion would not introduce too much energy gain or volume change. The structure will relax back to an ordinary rhombohedral one with the Jahn-Teller distortion disappeared. SCAN without U is able to keep the Jahn-Teller distortion, but not as large as those with self-interaction corrections.
In summary, distorting the initial structure (four-atom unit cell) to mimic the Jahn-Teller distortion, you will obtain the ground state one with DFT+U and/or HSE. Otherwise, you may miss it.
This may also needed for other transition metal compounds, so always double check you results and debug.